1. Field of the Invention
The present invention relates to a constant current source with threshold voltage and channel length modulation compensation, and more particularly to a current source that is applicable to digital-analog converter (DAC).
2. Description of the Related Art
DAC is the most commonly used circuit in integrated circuit (IC) design fields, and can usually be divided into active component type and passive component type. Passive DAC applies resistors or capacitors to complete such a circuit design. Because the passive components have a larger chip thereon, the matching between these passive components has to be taken into consideration. Furthermore, they need to be accompanied with high-efficiency operational amplifiers to have a good performance, so most current circuit designs don't adopt passive components and tend to adopt active components.
The active components generally can be divided into weighted current source, current cell matrix and switched-current modes in the design field of the DAC circuit. All of the above three modes of the active components have current sources formed by a plurality of current source cells, and make use of some switch components to switch current source cells so as to have various signal conversions.
As shown in FIG. 1, a circuit diagram of a conventional 10-digit DAC, the circuit adopts binary weighted current source for a design mode. The DAC includes 1023 current source cells 11 and ten weighted current source I0, 2I0, 4I0, . . . , and 512I0 formed by current source cells 11. The output signal can obtain 10-digit resolution by controlling the ten switches 12.
However, because the aforementioned circuit makes use of more than one thousand current source cells 11, the homogeneities of the current source cells 11 output current are very important; otherwise, it is impossible to obtain a DAC with a high resolution or high yield ratio.
FIG. 2 is a circuit diagram of a conventional current source cell. The output current I1 of a current source cell 20 can be formulated by the following formula:                               I          1                =                              K            1                    ⁢                                    W              1                                      L              1                                ⁢                                    (                                                V                  a                                -                                  V                  th                                            )                        2                                              (                  Formula          ⁢                                           ⁢          1                )            wherein K1=μnCox/2, μn is electron mobility, Cox is the capacitor value of the unit area, W1 is the channel width of a Metal Oxide Semiconductor (MOS) transistor M1, L1 is the channel length of the MOS transistor M1, Va is the bias voltage of the gate terminal and Vth is the threshold voltage.
From Formula 1, current I1 is variable with the threshold voltage Vth of the MOS transistor M1, so it is unacceptable for a high resolution DAC. In addition, not only the threshold voltage Vth may shift with the manufacture process conditions, but also the great current source cells of a DAC may have a poor PSRR (Power Supply Rejection Ratio; PSRR), which results in a distorted conversion.
To obtain a DAC with a better PSRR, another current source cell 30 is disclosed by Taiwan Patent No.230,284, as shown in FIG. 3. The output current I2 of the current source cell 30 can be simplified into the following formula:                               I          2                =                              K            2                    ⁢                                    W              2                                      L              2                                ⁢                                    (                              V                R1                            )                        2                    ⁢                      (                          1              +                              λ                ⁢                                                                   ⁢                                  V                  DS2                                                      )                                              (                  Formula          ⁢                                           ⁢          2                )            wherein K2 is a constant coefficient same in physics meaning as K1 in Formula 1, W2 is the channel width of MOS transistor M2, L2 is the channel length of a MOS transistor M2, VR1 is a first reference voltage; VDS2 is the relative voltage between the base electrode and source electrode of the MOS transistor M2, λ is a coefficient and the whole term (1+λVDS2) expresses the effect of channel length modulation.
Referring to the formula 2, because VR1 is a constant value, the output current I2 is in proportion to VDS2. However, VDS2 is also variable with the variance of the threshold voltage Vth of the MOS transistor M1. Compared to the current source cell 20 in FIG. 2, the relation between output current I2 and Vth is rewritten in the power of one from the power of two, so the PSRR of the current source cell 30 is likely to be slightly improved.
However, the current source cell 30 in FIG. 3 still cannot meet the requirements of a high resolution DAC. Therefore, a current source with a lower PSRR is progressively demanded for a DAC field to solve all aforementioned disadvantages in DAC.